I'm not a lawyer, but the terms and conditions seem to establish that there is minimum liability coverage provided where required.
For example, this Hertz T&C specifically calls out some minimum liability coverage, inclusion with the basic rental charge, and who it covers (bolding is modified by me).
- THIRD PARTY LIABILITY INSURANCE; INDEMNITY
(a) Hertz has a legal requirement to provide third party insurance coverage. This coverage is included in the basic rental charge.
(b) Hertz’ automobile liability insurance policy meets all legal requirements and protects Hertz, You and any Authorised Driver against legal claims from any other person for death or personal injury or
damage to any other person’s property caused by use of the Car.
(c) You agree to reimburse Hertz if Hertz is obliged to compensate (i) the insurers for any
payment they make to a third party on Your behalf and/or (ii) any third party, if that third party
suffers death, personal injury or damage to property caused by use of the Car by You or any
Authorised Driver in breach of paragraph 2 (Who May Operate the Car) or paragraph
5 (Prohibited Use of the Car) of this Agreement.
It isn't quite clear what the "legal requirement" is that they're meeting, but from a glance at Virginia's information, there is a requirement to carry the minimum and to extend it to anyone who operates the vehicle.
To purchase license plates or decals, you must certify that the vehicle is covered by the minimum insurance requirements
46.2-472.A.2. Insure as insured the person named and any other person using or responsible for the use of the motor vehicle or motor vehicles with the permission of the named insured.
There is slightly different language in this Hertz rental agreement, so I would review your own carefully before deciding.
11.1 - We have a legal requirement to ensure that third party insurance coverage is provided. This coverage is provided by us and included in the rental charge unless we agree with you that your own insurance will cover the rental.
The minimums are also very low. It's not hard to imagine a scenario where they could be exceeded, and without any personal auto insurance backing you up, it may be worthwhile to pay for the liability supplement.
46.2-472.A.3. ... $30,000 because of bodily injury to or death of one person in any one accident and, subject to the limit for one person, to a limit of $60,000 because of bodily injury to or death of two or more persons in any one accident, and to a limit of $20,000 because of injury to or destruction of property of others in any one accident.